Introduce simple approach to command line handling

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Support reading mailboxes from stdin

This mode is enabled in two cases:

  1.) There is _no_ non-option argument on the command line
  2.) There is exactly _one_ non-option argument and that
      argument is exactly one single dash: "-"

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>